In recent years, many of aluminum-containing metallic heat exchangers are, in order to improve the heat dissipation effects or cooling effects of the exchangers, designed so that the area of the heat dissipation section and cooling section is large as much as possible, and for this reason, the fin spacing is extremely reduced. In addition, in order to reduce the ventilation resistance as much as possible, notches referred to as louvers are put between fins in some cases. For this reason, when the exchangers are used as coolers, moisture in the atmosphere is condensed on the surfaces of the heat exchangers, in particular, between the fins, and the condensed water is more likely to become water droplets as the fin surfaces are more hydrophobic, and causes clogging in fin gaps to cause a problem of increasing ventilation resistance, thereby decreasing heat exchanger effectiveness.
Furthermore, the water droplets condensed between the fins induce corrosion of the aluminum forming the fins to deposit an aluminum oxide in white powder form on the fin surface, and makes bacteria, etc. more likely to grow on the surfaces when the surfaces of the heat exchangers left for a long period of time while remaining wet. The white powder of aluminum oxide deposited on the fins and the water droplets retained in the fin gaps fly in a room through blowers of the heat exchangers to provide a feeling of discomfort to users.
In order to prevent the clogging from being caused by the water droplets, treatment methods of providing fin surfaces with hydrophilicity to improve water wettability have been developed. In this case, for the fin surfaces, anticorrosion films (for example, chromate phosphate films, chromate films by chromic acid, or non-chromate films) are first formed on the heat exchangers. Then, on the anticorrosion films, hydrophilic inorganic compounds such as water glass and colloidal silica, as well as organic compounds such as surfactants and water-soluble resins are used singly or in combination to form hydrophilic films. It is to be noted that hydrophilic films may be directly formed without forming the anticorrosion films in some cases.
For example, JP H9-296121 A discloses a hydrophilic treatment agent for metal materials, which contains a water-soluble polymer having an acetal structure and a hydrophilic polymer having at least one or more hydrophilic groups.
JP H11-343450 A discloses a composition for hydrophilic treatment, which contains a water-soluble polymer, a trivalent chromium compound, a zirconium or titanium compound.
However, the hydrophilic films are not always considered to have excellent drainage performance, but the development of hydrophilic films which have further excellent drainage performance has been desired. Moreover, the development of hydrophilic films which stably provide favorable drainage performance for a long period of time has been desired, because the hydrophilic films undergo a significant decrease in drainage performance after use with time.
Furthermore, WO 2009/044912 A discloses a hydrophilic coating agent containing a colloidal silica sol (A), an acrylic polymer (B), a reactive silane coupling agent (C), and a curing agent, characterized in that the mass ratio {(A)/(B)} is 5/95 to 95/5, and the ratio {(A+B)/(C)} is 30/70 to 95/5.
The hydrophilic films exhibits anti-fog properties because of their high hydrophilicity, hardness, durability, but fail to provide adequate drainage performance.
JP 2005-162533 A discloses a method for producing modified colloidal silica, which is characterized in that colloidal silica obtained by hydrolysis and condensation of a hydrolyzable silicon compound is modified with a modifying agent.
The production method is aimed at modifying the colloidal silica for the improvement of dispersion stability for a long period of time. However, in this method, when the mass ratio of a silane coupling agent exceeds 0.03 with respect to the colloidal silica, problems are caused such as increased secondary particle size, formation of aggregates, and gelation, and even if this silica is used for a hydrophilic surface treatment agent, the amount of the silane coupling agent (it is to be noted that organo alkoxy silane is a type of silane coupling agent) with respect to the colloidal silica does not correspond to adequate surface modification, which is insufficient for providing drainage performance.
JP H7-233270 A discloses an agricultural film which is excellent in antifog properties, which is characterized in that the film is obtained by the emulsion polymerization of an acrylic monomer, a polymerizable silane coupling agent, and colloidal silica, and provided with a coating layer containing 20 weight % to 70 weight % of colloidal silica per total solid content.
The method mentioned above can provide the film surface with permanent hydrophilicity and water droplet flowability, while maintaining excellent transparency and durability at the surface of the agricultural film for use in vinyl greenhouses for agriculture, etc. However, also in this method, the ratio by weight of the silane coupling agent with respect to the colloidal silica is 0.5 at a maximum, which means that the surface modification with the silane coupling agent is insufficient. In the method, the silane coupling agent is used which has a hydrophobic group in the molecule, and the silica surface will be covered with the hydrophobic coupling agent when the agent is contained at 0.5 or more in terms of ratio by weight, and the hydrophilicity of the silica will be thus lost to cause a problem of failure to obtain hydrophilicity at the surface of the coating film. Therefore, in the case of the agricultural film or the like there is no problem with water droplet flowability, while the method is inadequate for drainage performance of structurally complex aluminum-containing metallic heat exchangers.
JP 2001-323250 A discloses a hydrophilic treatment agent containing silica microparticles and a vinyl alcohol polymer, where the silica microparticles are coated with the vinyl alcohol polymer, and dispersed in an aqueous medium in the state of coated particles of 5 nm to 1000 nm in average particle size.
The method mentioned above provides a hydrophilic treatment method for heat exchangers, which is excellent in maintenance of hydrophilicity and deodorant properties. However, even when the silica microparticles are coated with the vinyl alcohol polymer, the drainage performance is inferior to that of colloidal silica surface-modified with a silane coupling agent while the silica microparticles are excellent in deodorant properties.
JP H6-93209 A discloses a hydrophilic film forming agent for aluminum exchangers or fin materials thereof, which contains, as its main constituents, an alkali silicate, a low-molecular organic compound having a carbonyl group, an acrylamide copolymer, and a silane coupling agent, with the solid content of the acrylamide copolymer from 25 weight % to 30 weight %.
The method mentioned above provides hydrophilic films which can reduce cement odors and mold odors specific to the alkali silicate, but the films also have inadequate drainage performance.
Patent Literature 1: JP H9-296121 A
Patent Literature 2: JP H11-343450 A
Patent Literature 3: WO 2009/044912 A
Patent Literature 4: JP 2005-162533 A
Patent Literature 5: JP H7-233270 A
Patent Literature 6: JP 2001-323250 A
Patent Literature 7: JP H6-93209 A